I have a pretty vanilla website with a simple paypal integration for selling tickets. 363 days of the year it gets very little traffic. However, the two days a year when sign ups happen I get a spike. I have around 250 sessions at a certain time when sign ups are enabled. Many users complain of 504 and other errors. Many users are still able to complete sign ups.
Any tips or tricks to help alleviate some of the issues under load? I have enabled caching plug-ins previously, but currently don't have any enabled. Would caching help? Any recommendations for caching plugins? Is this a problem with my plan or a limitation of wordpress? Or the way I implemented the site with the current plugin?
This could be an issue with the concurrent connections limit on mysql. If you have too many users the checkout may be timing out as a result of the stalled connections building up and taking too long to process.
You may want to consider a vps where you have control over something like that by optimizing mysql.
Its a lot of work to move a wordpress site to another server temporarily but it can be done relatively easy if you know what you are doing.