My first manga, it was normal to take two hours for four panels.
Two things that made it slower:
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I had to look up all the kanji.
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It was a 4koma, so almost every panel had dialogue, and often a lot of dialogue.
For something like Takagi-san, I think one hour per page sounds about right.
The grammar probably will be the most time-consuming part. But that gets faster over time, as you learn grammar (read the meaning) and then you acquire grammar (understand from seeing it in context repeatedly).
あとは?
At a basic level, は shifts the topic (and as such, the context) of the comment.
The comment is the portion after the topic+は.
The prior topic/context was on the spring and he’s shifting the topic/context to what comes after the spring. He’s not actively thinking about changing the topic; it’s a natural part of Japanese speech.
完成 is a noun meaning “complete” or “perfect”.
An example in English would be “My homework is complete.” (There is no verb/action here.)
If you simply use the word 完成 by itself, that is very ambiguous. There are many things that can be 完成. You can narrow it down by presenting a topic, or by specifically naming a subject. Or you can modify 完成, which is what is happening here. 完成 is being modified by ビックリ箱 to refer to a complete ビックリ箱.
In this case, the topic is set to あと. And the subject is unstated, so we can use “it” for the subject in English. He’s essentially saying “(I’ve finished the spring and) with that it is the completion of the jack-in-the-box.”
I’ve seen it said that this use of さ at the end of a sentence makes the statement “playfully assertive”, which I think describes it nicely.
but I’ll get there eventually 
)
