That is a personal decision. You can find many financial articles using Google with the pros and cons. It usually depends on your age and how close to retirement you are - the younger then the more the conversion will pay off. Also, there is a big hit if you pay the tax from the IRA since that reduced the amount available for future growth. It is much better if you can pay the tax from other funds and convert the entire amount. You can also convert a portions each year to reduce the tax in any one year.
If you are unhappy with your investment performance in your IRA, that is mainly a result of the securities you have chosen to invest in in the market. There is no magic solution to moving that money into a different kind of IRA or 401(k).
There are a few possible advantages of rolling an IRA into a 401(k), if your plan allows it, but there are also some disadvantages. One advantage is that 401(k)s often have access to funds with lower expense ratios than private investors have access to. On the other hand, money in the 401(k) (generally) can’t be withdrawn unless you terminate or retire from employment with the plan sponsor, whereas money in an IRA can be withdrawn at any time, although you may pay a penalty. Here’s an article summarizing the differences. https://www.nerdwallet.com/blog/investing/rollover-ira-to-401k/
The interesting thing about converting a traditional IRA to a Roth IRA is that you pay the taxes now and that takes the government out of the equation in the future. If you think that income tax rates will increase over time, then converting to a Roth now will avoid that tax increase. Of course, you must have the funds available to pay the tax now, because any money that you hold out to pay taxes will be counted as an early distribution and will be subject to income tax plus the penalty. You could transfer a little bit at a time, you don’t have to transfer it all at once.
But no particular investment vehicle is going to be a magic cure for a down market.
If you have a 401(k) at your new job, you might consider investing in an after-tax (“Roth“) 401(k) plan instead of a pretax 401(k). For every $100 that you might invest pretax, you can probably only afford to invest $65 after tax, but you will not owe any tax in the future on the gains. (I tried to do some tests a while ago, and the smaller net investment combined with no tax on withdrawal seems to be a wash compared to the larger net investment with taxes due, as long as income tax rates stay the same. If they go up, after-tax investing will probably be a net gain.)
And I think there are some other arguments in favor of having a mix of pretax and after-tax assets available to you when you enter retirement.