similar variables detector is extremely slow

This detector makes up the majority of runtime on large codebases

ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000   41.286   41.286 /Users/alpharush/tob/slither/slither/__main__.py:81(process_all)
        1    0.000    0.000   39.059   39.059 /Users/alpharush/tob/slither/slither/__main__.py:58(process_single)
        1    0.000    0.000   33.319   33.319 /Users/alpharush/tob/slither/slither/__main__.py:111(_process)
        1    0.000    0.000   33.317   33.317 /Users/alpharush/tob/slither/slither/slither.py:198(run_detectors)
        1    0.000    0.000   33.317   33.317 /Users/alpharush/tob/slither/slither/slither.py:204(<listcomp>)
       84    0.001    0.000   33.317    0.397 /Users/alpharush/tob/slither/slither/detectors/abstract_detector.py:176(detect)
        1    0.000    0.000   31.215   31.215 /Users/alpharush/tob/slither/slither/detectors/variables/similar_variables.py:72(_detect)

https://github.com/crytic/slither/blob/master/slither/detectors/variables/similar_variables.py#L63-L66