Why do some calls to findNonZero with UMat take more than 500 msec?

My first foray into using a GPU has been disappointing due to weird timing results. Below is a snippet of code using UMat in OpenCV to find dead-space at the top and bottom of a binary image. Most of the time the findNonZero call excutes in less than one msec, but occasionally it takes more than 500 msec! The delay seems independent of the size of the result. Can someone offer an explanation and fix?

    UMat bin; 
    // bin is loaded with a binary image of about 60 x 60;

    int top = bin.rows;
    int bottom = 0;
    for (int i = 0; i < bin.cols; i++)
        UMat r = bin.col(i);
        vector<Point> pxls;
        findNonZero( r, pxls);
        cout << pxls << endl;
        if (!pxls.empty())
            if (pxls.front().y < top) top = pxls.front().y;
            if (pxls.back().y > bottom) bottom = pxls.back().y;

Here is the report on my OpenCL support:

1 GPU devices are detected.
name:              AMD KAVERI (DRM 2.50.0, 5.8.0-36-generic, LLVM 11.0.0)
available:         1
imageSupport:      0
OpenCL_C_Version:  OpenCL C 1.1 

Number of platforms                               1
  Platform Name                                   Clover
  Platform Vendor                                 Mesa
  Platform Version                                OpenCL 1.1 Mesa 20.2.6
  Platform Profile                                FULL_PROFILE
  Platform Extensions                             cl_khr_icd
  Platform Extensions function suffix             MESA

  Platform Name                                   Clover
Number of devices                                 1
  Device Name                                     AMD KAVERI (DRM 2.50.0, 5.8.0-36-generic, LLVM 11.0.0)
  Device Vendor                                   AMD
  Device Vendor ID                                0x1002
  Device Version                                  OpenCL 1.1 Mesa 20.2.6
  Driver Version                                  20.2.6
  Device OpenCL C Version                         OpenCL C 1.1 
  Device Type                                     GPU
  Device Profile                                  FULL_PROFILE
  Device Available                                Yes
  Compiler Available                              Yes
  Max compute units                               8
  Max clock frequency                             720MHz
  Max work item dimensions                        3
  Max work item sizes                             256x256x256
  Max work group size                             256
  Preferred work group size multiple              64
  Preferred / native vector sizes                 
    char                                                16 / 16      
    short                                                8 / 8       
    int                                                  4 / 4       
    long                                                 2 / 2       
    half                                                 0 / 0        (n/a)
    float                                                4 / 4       
    double                                               2 / 2        (cl_khr_fp64)
  Half-precision Floating-point support           (n/a)
  Single-precision Floating-point support         (core)
    Denormals                                     No
    Infinity and NANs                             Yes
    Round to nearest                              Yes
    Round to zero                                 No
    Round to infinity                             No
    IEEE754-2008 fused multiply-add               No
    Support is emulated in software               No
    Correctly-rounded divide and sqrt operations  No
  Double-precision Floating-point support         (cl_khr_fp64)
    Denormals                                     Yes
    Infinity and NANs                             Yes
    Round to nearest                              Yes
    Round to zero                                 Yes
    Round to infinity                             Yes
    IEEE754-2008 fused multiply-add               Yes
    Support is emulated in software               No
  Address bits                                    64, Little-Endian
  Global memory size                              2142642176 (1.995GiB)
  Error Correction support                        No
  Max memory allocation                           1499849523 (1.397GiB)
  Unified memory for Host and Device              No
  Minimum alignment for any data type             128 bytes
  Alignment of base address                       32768 bits (4096 bytes)
  Global Memory cache type                        None
  Image support                                   No
  Local memory type                               Local
  Local memory size                               32768 (32KiB)
  Max number of constant args                     16
  Max constant buffer size                        1499849472 (1.397GiB)
  Max size of kernel argument                     1024
  Queue properties                                
    Out-of-order execution                        No
    Profiling                                     Yes
  Profiling timer resolution                      0ns
  Execution capabilities                          
    Run OpenCL kernels                            Yes
    Run native kernels                            No
  Device Extensions                               cl_khr_byte_addressable_store cl_khr_global_int32_base_atomics cl_khr_global_int32_extended_atomics cl_khr_local_int32_base_atomics cl_khr_local_int32_extended_atomics cl_khr_int64_base_atomics cl_khr_int64_extended_atomics cl_khr_fp64