Done with slow focus

To make it short: AF has improved in the latest models, whatever the brand.

Best AF performance is an expensive differenciating feature that is offered only in top end models

Pentax AF also improved, but Pentax AF-C tracking is still behind Nikon and Canon best cameras.

When comparing with a smartphone or a small sensor compact, dont forget that the depth of field of the smaller sensor is dramatically larger: shooting at f2.8 with a small sensor compact or a smartphone, you will get the same depth of field as when shooting f11 in APS-C. As smartphone only have a wide angle lens, almost everything will be in focus, you loose the bokeh and 3D effect, but it is very forgiving for the AF.

In details:

I have K30, which is same camera as K50, since february 2014.

With K30, the only way to track a moving target in AF-C is to use single point AF and pan the camera to track the target yourself: the camera AF will work only in the Z axis, and it does that well. What it doesnt do well is tracking the target in x and y axis.

On K5/K30/K50, if your target is small, it is better to use only the central AF point, which is cross type and more precise than the other AF points. Other AF point will be prone to mistake on what is exactly your target, pecularly if the background is busy.

That is why last year, after trying my son's K3, I too bought a K3 for its much improved AF.

The 25 cross type AF points in K3/K3-II and KP are much more precise than the 9 points in K5/K30/K50, and I can track moving target in the Z axis with any of them.

Yet the K3 struggles to track by itself in X and Y axis. It is the only true limitation of Pentax cameras when compared with Canon and Nikon DSLR or most recent high end mirrorless.

You asked about the DA 18-135. I have it and it is my most used lens. It wont AF faster on a static target than the DA50 f1.8, but, IMO, its AF-C seems a little more efficient on a moving target. But as DA 18-135 at 50mm is only f4.5, the shallower depth of field when shooting at wider apertures with DA50 f1.8 can also explain a lower hit rate.

The two samples below have been shot during a Carnaval parade in single point AF-C with DA 18-135 fully open to get some background blurr :

They were walking steadily, but very close to me

As these dancers moved faster and were also quite close, I tracked the lady in the foreground  with a rather wide framing, and cropped the shot to my taste in PP, as all AF points are clustered in the center of the frame. That is why, though the K3 has 24MP, there are only around 12MP in the final picture

When comparing with a M43 camera, dont forget that the depth of field is larger on the smaller sensor: it is like shooting at an aperture value one stop closer on APS-C cameras.

I also have a M43 Panasonic GM5, which is very snappy to AF, even in dim light, but not good to track in the X and Y axis.

Also ensure that the AF in your K30 has been fine tuned for every lens you use, and pecularly for large aperture lenses like your DA50: read the manual to understand how to do it.

For ultimate precision, you need to use a tripod and a special AF target, but you can do a very efficient quick fix handheld by shooting with the lens fully open to a far away static target (a textured building about 30 meters/yards away is a good target for this purpose) and comparing the focus both in live view and using the viewfinder PDAF.

The liveview AF works the same as in mirrorless cameras, by maximizing the contrast on sensor: hence, it is always accurate.

But when using the viewfinder, AF is done by a dedicated sensor, as the image sensor is hidden by the mirror.

This is why it may often needs to be adjusted for each lens, using the camera dedicated feature: the focus accuracy through the viewfinder should be the same as in liveview.

If not, you enter a correction value and try again until you reach the value that delivers the best results. Then try again on a close distance target, usually it will be OK. If it needs a different correction, it is up to you to choose which intermediate value suits you best.