This can be due to multiple reason and can impact cumulatively.
Analyses below stated and points and act on each.
Important one as per your screen posted.
1. Low quality of hosting, VPS:
2. Run in Production mode.
3. Check JS overall required on checkout,lower the JS size better it will be.
4. Turn on cache( I expect this is in place).
5. Analyses modules installed that are impacting/extending your checkout.
6. HTTP 2 and no JS merge.
Other points to be looked at
7. How far is the payment gateway server from your hosting location.Should be in same country.
8. Upgrade Magento version to 2.3.x.
9. Clean data base for old quotes and logged data that are not of much use.
Questions: are you using Magento default checkout or have you installed some checkout module.