Web Designer is fast, I use a mac with fiberlink 200Mbits with Chrome to develope Powerapps, I found The "visual studio" Windows 10 is slower that web access, and Mac scrolling on web is a lot nicer and smoother that with Windows Explorer where you scroll x line by x lines and not pixel by pixel.
I have not experimented any response times for clicks or typing, except may be some time in deleting things.
The big problem with designer is that you cannot have you own templates for screens, gallery ou datacard and you need to do it again and again for each occurence
Also there is no Responsive Design Option and you have to position your controls manually (no way !) or use variables like margin/top/botom/left/right and pout thoses variables in XY heigth width related to next controls for each control which is time consuming. They have invented rules to help but not to my satisfaction.
There is no problem in SQL Server for the number of Tables, 100 is still a few. But if your database is on the web you will get slower response time than local especialy with large transfer file responses with slow connection.
In fact Access Web App uses Azure SQL Server as a Database ! so you will get the same response time that Powerapps 🙂
Access will be faster only with a local implementation.
For the table think of Customer/People/Offers/Actions all with a link to all others that is 4*3*2 links, for the lookups you store a code and a libelle in a table ie: AC: Active Customer, P Prospect, PA Partner etc... you have the code in the Customer Table (PA) and you need to lookup the libelle in order to display it (Partner), if you have 30 lookups with 200 char each and yout tables have 20000 customer with 60000 names you spare 30*200*70000= 420 megabytes, I agree this is an old habit from when megabytes were expensive but Another reason is that you can modify the libelle without updating all records, and finaly thimk that you need 2 languages or ..50.. Text should be separated to codes anyway.