I don't remember getting much to do with maths in the air with my instructors
The OP actually said 'mental arithmetic' not 'arithmetic in the air' - and I can, now, think of one calculation he should be able to do at 10 hours:
"OK Pat you say we have 40 Litres of fuel on board. Is that total or usable?
This aircraft uses approximately 25 litres per hour. For today's exercise we will be flying for approximately one hour. As you know we like to land with a least 45 minutes worth of fuel remaining. Can we go, or do we need to go to the Pumps first?"
The above is necessary, but it doesn't have to be done by mental arithmetic - just create a crib card.
Of course, in the air (eg during Cruise Checks) you should, by now, be able to mentally calculate roughly how much time you have left.
Checks in the air do need to be done in a timely manner, but that comes with practice.
Checks on the ground should never, ever be rushed.
Nor should an Instructor suggest to a student to hurry up.
However: A Preflight Check, which could easily be done by a PPL in 5 minutes might take a brand new student 30 minutes. If, after 10 hours it is still taking 30 minutes (not having even reduced to 25 minutes) then the Instructor would be correct to comment on this to see what the cause might be.
The OP mentioned a lack of continuity (due Wx) and this could be the reason. So I repeat my previous suggestion: Have a lesson on 'Checks'. No flying involved so can be done on one of those bad Wx days.