there are many reasons that sql server/MySQL (which one by the way?) can be slower
The decision was made to consolidate our largest and most used databases into one front end and one SQL server for the data.
this is one of them - if many people are using the server - different apps perhaps - then the server can be slower. It needs to be sized and tuned. Indexes in particular need to be rebuilt on a regular basis on tables subject to many changes - similar to compact and repair.
A client of mine migrated the backend to sql server thinking it would be faster - in fact it was significantly slower, despite optimisation. Reason as above. One app (not access) was such a heavy user that all the other apps suffered. IT did not have the budget to increase resources and in the end, the backend was returned to the ACE environment.
with regards your form and optimisation, consider whether you have addressed the issues raised in this link https://www.access-programmers.co.uk...hlight=indexes