How can I improve my online store's mobile speed?

I recently hired a shopify “expert” MS Designer to improve my speed scores. While they were able to improve the PC speed, they could not do anything about mobile and claim the reasons they can’t are too complicated to explain. They are refusing to refund me 1/2 of what I paid saying they can’t guarantee anything… What a rip-off!! They wrote that my mobile was at 60, but I checked it and it was still only 43. I wrote to Shopify Customer Service again and did not receive a response. Buyer beware!!

@JK2020 ,

Please share the store URL.. so that I can check.