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Several things to try:
Instead of foreach loops, switch them out for 'for' loops, they have been proven to be faster: https://www.c-sharpcorner.com/article/for-vs-foreach-in-c-sharp/
I'd look into concatenation of string more efficiently: https://stackoverflow.com/questions/21078/most-efficient-way-to-concatenate-strings
I'd replace all instances of ...
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