Make Timer function more efficient

I am running a macro every time increment I want, can change that increment in B1.

My problem is that I run this on a workbook with LOTS of calculations that take place every second and every time my macro runs after it is called by the timer, excel struggles and takes a couple of seconds to calculate. If I turn off automatic calculation for the full workbook, everything works fine! But I CANNOT turn it off permanently because I still need all of these other calculations to run in the meantime.
I tried to use Application.Calculation = xlManual & Application.Calculation = xlAutomatic at the beginning and the end of macro1 but it would not change anything. I know it's coming from the start1 macro but I am not sure how to tweak it to make it work. Basically I think I need to change to Manual calc every time I reach the time increment and change back to Automatic right after...

Option Explicit
Public Increment As Double
Public updatetime As Date



Sub macro1()


Increment = Sheets("Sheet1").Range("B1").Value

Sheets("Sheet1").Range("C1").Value = Sheets("Sheet1").Range("C1").Value + Increment

Call Start1

End Sub

Sub Start1()


    Increment = Sheets("Sheet1").Range("B1").Value
    
    updatetime = Now + TimeValue("00:00:" & Increment)
    
'    Application.Calculation = xlManual

    Application.OnTime updatetime, "macro1", Schedule:=True
'    Application.Calculation = xlAutomatic

    
End Sub

Sub Stopit()
    On Error Resume Next
    
    Application.OnTime updatetime, "macro1", Schedule:=False
    Range("C1").Clear


End Sub