Are you just selecting everything on the foundation level and using an SEO with upwards extrusion? If so, alternatively, you might try using a single (or a couple, if necessary) slab at the lowest level around the foundation perimeter, so you are using maybe less than 5 SEOs total. You can keep the original model for cut & fill calculations, but for day-to-day working, you'll have a faster model. You can touch up sections with 2D fills, if necessary.
Richard
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Richard Morrison, Architect-Interior Designer
AC26 (since AC6.0), Win10