GEMSIM takes noticeably longer to solve models than a TABLO-generated program. This is especially true for larger models.
To see a comparison of solving times for some popular models, please see here.
Note that Executable-Image Versions of GEMPACK always use GEMSIM to solve models while with a Source-Code Version of GEMPACK it is usually quickest to use a TABLO-generated program to solve large models (but GEMSIM may still be used if preferred).