I can chime in here
DHL (we also offer UPS, but DHL is nearly always cheaper and better, plus we discount the rate by 25%) is by far the most reliable option, but much more chance of getting dinged with duty/tax. We do what we can to minimise it (), but since all the customs info is sent to DHL and UPS over the wire, there's less chance of it sliding under the radar.
On the other hand, we have ye olde postal services. We use a hybrid option which should speed things up a bit, but, all the postal services (USPS and Royal Mail in this case) are reliant on commercial airlines for the trip across the pond, so that's where the delays can happen. It's much better than it was, but capacity is still 40% of what it would normally be.
From here in Texas we ship UPS Ground to a sorting center in New York, and that's when it's handed off to the postal service for onward shipping and delivery. The advantage of this option is that it's cheap, and since most of what we ship is lightweight, they stand a a much better chance of flying under the customs radar. We still complete a declaration of course, but unlike the faster services, it's stuck to the outside of the package for manual review.
At the height of the Pandemic, the slower/cheaper option was really slow, at one point the postal service was using ships to clear backlog as there was just no airlift available. The backlogs in the sorting centers was what caused us to switch hubs from Chicago to New York. That's no longer the case, so we're seeing an average of ~14 business days for the slow option, and nearly always 3-4 days for DHL. There are outliers though, and if a hub or sorting center gets hit with Covid issues, it can cause a lot of disruption.
One final point, we do not currently include the duty/VAT charge in the shipping cost. It's fairly complex to do that (varies by product and country), but if we cannot sort out local distribution soon, that'll be plan B.
Always around if there's any questions, might just have to @mention me to get my attention.
